This invention relates generally to battery rechargers and, more specifically, to a battery recharger that uses non-dissipative voltage equalization of series connected battery cells, typically lithium ion cells, during recharging.
Rechargeable batteries are well known for their versatility of being a readily replaceable power source that allows portability. Several drawbacks exist in most battery recharging systems. First, there are various types of rechargeable batteries ranging from standard lead-acid batteries to new exotic nickel metal hydride and lithium ion cells. In all cases, overcharging of the cells during the recharge is detrimental to the batteries in that excess heat and/or pressure may occur. This excess heat may destroy the battery by either damaging the battery structure or altering its chemical composition thus limiting its recharging capacity. Over pressure developed during overcharge can lead to nickel-cadmium cells exploding, which not only destroys the battery, but may release cadmium, which is adverse to the environment.
New systems of rechargeable batteries have recently been brought to market. These systems use lithium ion cells, which have advantages over other forms of rechargeable battery technology in that they have much higher charge and voltage capacities over conventional rechargeable batteries. Moreover, they are able to maintain their charge during a longer state than the other rechargeable systems. For example, ni-cad batteries are used with many electronic systems; however, ni-cad systems lack the ability to maintain charge for extended periods between use as well as to deliver a high charge for any length of time during operation.
Accordingly, several ni-cad cells must be combined in order to provide adequate powering capabilities in most electronic systems. Lithium ion cells, on the other hand, have overcome many of the disadvantages that are associated with ni-cad cells by being both more efficient and lighter than other forms of rechargeable batteries. This means that portable batteries for consumer electronic products can now be powered by batteries that are more efficient, lighter and more convenient.
Lithium cells are not without certain disadvantages, however. One drawback limiting the broader use of lithium cells is their susceptibility to overcharge. This is especially so when more than one battery is to be charged with the same charger. Overcharging can result in dangerous over temperature and over pressure levels that can potentially damage the cell and the electric system. One solution to prevent overcharge is to monitor the charge level on the cells such that as soon as one cell reaches a preset level, charging stops, leaving the remaining cells without a full charge.
Accordingly, what is needed is a battery recharger that is capable of distributing charge among several batteries being charged simultaneously without over charging or applying over voltage to any one or all of the cells. The system also should be able to recharge all batteries in the system to their full potential.